1. Field of the Invention
The present invention relates to a method for operating an electrographic printer or copier in which toner particles of different colors are deposited onto a toner image carrier by at least two developer units.
2. Description of the Related Art
When the developer units in an electrographic printer have to develop two charge images with different colors in succession, where the regions to be developed in the two charge images have the same potential, one of the two developer units must be selectively switched into a transfer mode in which toner particles are deposited on the toner image carrier. The other developer unit, or respectively, the other developer units must be in a resting state in which no toner particles are deposited on the toner image carrier. It must be guaranteed that a developer unit deposits toner particles onto the charge image only when the charge image is to be developed with toner particles of this developer unit. It is therefore necessary that the developer units with toner particle of a color that is not needed remain in the resting state when the charge image is moved past them. This is true regardless of whether the charge image has already been developed with toner particles or it will be developed with toner particles at a subsequent developer unit. The toner image carrier is a photoconductor, for example, from which the toner image is directly or indirectly transferred, via a transfer band, onto a final image carrier, such as paper.
A known solution according to a first embodiment in European Patent Document EP 0 691 586 A1 provides that the developer units are swivelled away from the toner image carrier in the resting state, in order to prevent a transfer of toner particles of the wrong color. This method is disadvantageous in that the developer units must be swivelled relatively far away in order to switch over from the transfer state to the resting state. Accordingly, in the changeover from the resting state into the transfer state, the developer units must be swivelled relatively far again back to the toner image carrier. The changeover thus cannot be executed arbitrarily rapidly. Besides, the motorized swivelling of the developer units is only possible with a plurality of additional mechanical components, which are subjected to heavy wear. Often, in addition to swivelling, other measures must be taken, such as an activation and deactivation of rotating components in the developer unit, in order to prevent a dusting or undesired spreading of the toner. This dust would lead to undesirable background formations on the toner image carrier and to contamination of the printer. The activation and deactivation of the rotating components further prolongs the time for the changeover. The European Patent Document EP 0 691 586 A1 also describes a second embodiment, in which a force field in the development gap is activated, or respectively, deactivated in order to switch on, or respectively, switch off the developer unit.
In a developer unit according to U.S. Pat. No. 5,270,782, in the off state of the developer unit, the toner particles on the developer roll are removed by means which are not further detailed.
The above described problems arise in electrographic, magnetographic and ionographic printers or copiers, among others, which fall under the heading of electrographic printers. The toner used is either a single-component or multi-component toner. Solid or liquid toners may be used.
According to German Patent Document DE 4113 777 A1, charge regions of equal potential can be developed with toner particles of different colors if an exposure unit is allocated to each developer unit. A developer unit does not develop if the allocated exposure unit is switched to dark. The above problems with respect to the activation and deactivation of the developer units do not arise. However, for this, a plurality of exposure units must be installed in the printer, which units require space and which entail expensive manufacturing technology.